The invention relates to improvements in catalytic converters for optimizing the ratio between pressure drop and mass transfer of gases.
Usually, a catalytic converter has a substrate constructed from a considerable number of neighboring small channels through which flows a gas or a gas mixture to be converted by means of a catalyst coated on the substrate. Various materials may be used to construct catalytic converters, such as ceramic materials, or metal, e.g. stainless steel or aluminum.
The cross-section of the channels of ceramic catalytic converter substrates usually is rectangular or polygonal, i.e. hexagonal. This type of catalytic converter is manufactured by extrusion, a method that produces channels having the same cross-section along their entire length and channel walls that are smooth and even.
In the manufacture of catalytic converter substrates from metal, corrugated strips or foils are arranged alternatingly with flat strips or foils and such an assembly is wound about an axis. A resulting channel cross-sectional shape is triangular or trapezoidal. The metal catalytic converters that are available on the market are formed with channels having an equal cross-sectional size along their entire length and, like the ceramic catalytic converter substrates, their channel walls are smooth and even.
The most essential feature is the mass transfer that takes places between the gas (or the gas mixture flowing through the channels) and the catalytic converter channel walls. The coefficient of mass transfer, which is a measure of the mass transfer rate, must be high if high efficiency of catalytic conversion is to be achieved.
In catalytic converters of the kind mentioned above, which are used in internal combustion engines or in industry, the channels have a comparatively small cross-sectional shape, and the gas, at the gas velocities common in these contexts, flows in comparatively regular layers in the direction of the channels. Thus, the flow is-mainly laminar. Only along a shorter length adjacent the channel inlets does some cross-wise flow take place in the direction towards the channel walls. To categorize the gas flow the so called Reynolds number is used, the value of which in these applications is between 100 and 600. As long as the Reynolds number remains lower than approximately 2000, the flow remains laminar.
It is well known within the technical field concerned, that in laminar gas flows a boundary layer is formed closest to the channel walls, in which boundary layer the gas velocity is substantially zero. This boundary layer strongly reduces the coefficient of mass transfer, above all in the case of so called fully developed flow. In order to increase the coefficient of mass transfer, the gas must be made to flow towards the channel surface, which reduces the boundary layer and increases the flow transfer from one layer to another. This may be effected by turbulent flows. In smooth and even channels the laminar flow turns turbulent when the Reynolds number reaches values above approximately 2000. If one wishes to reach a Reynolds number of this magnitude in the channels of the type of catalytic converters concerned herein, considerably higher gas velocities than are conventional in these contexts are required. In the catalytic converters of the kind referred to above having a low Reynolds number, it is therefore necessary to create turbulence by artificial means, for instance by arranging special turbulence generators inside the channels.
A large number of turbulence generators are already known. From European Publication No. 0298943 there is known a catalytic converter having channels with turbulence generators therein in the form of transverse corrugations. From Nonnenmann et al. U.S. Pat. No. 4,152,302, there is known a catalytic converter having channels in which turbulence generators in the form of transverse metal flaps punched from the structural material are provided. Also combinations of these two types of turbulence generators exist.
A feature common to turbulence generators of this kind is their ability to significantly increase the mass transfer. However, also the pressure drop increases dramatically. In fact, the pressure drop increase has proved to exceed the increase of mass transfer. The pressure drop as such depends on the configuration, dimensions and geometry of the turbulence generators. However, it is well known that said types of turbulence generators produce a pressure drop that is too high, which has prevented them from being used commercially to any significant extent.